Foraging behaviour in relation to the lunar cycle by Australian Owlet-nightjars Aegotheles cristatus

The purpose of our study was to assess the influence of moonlight on the fo raging behaviour of Australian Owlet-nightjars (Argotheles cristatus), an A ustralasian caprimulgiform bird. Australian Owlet-nightjars should face a d istinct tradeoff between increasing activity during moonlit nights to enhan ce foraging, while at the same time risking increased exposure to visually- oriented nocturnal predators. Out objectives were to quantify foraging acti vity in relation to moonlight using radio telemetry and document the princi pal foraging strategy of Australian Owlet-nightjars by observing light-tagg ed, free-ranging birds. Our results showed that like true nightjars, Austra lian Owlet-nightjars are almost exclusively sally-type foragers and rarely forage on the ground. They suffer high rates of predation relative to other adult caprimulgiforms and are active almost exclusively during true night. Overall, activity levels inferred from telemetry data declined over the co urse of the study (September-March), although this effect was overridden on Full moon nights when there was no seasonal effect and activity levels rem ained constant. Unlike true nightjars for whom bright lunar light enhances foraging activity, our results for Australian Owlet-nightjars suggest that the risk of predation influences foraging activity, especially on full moon nights. Although Australian Owlet-nightjars apparent ly employ the same fo raging mode as nightjars do, knowledge about foraging strategy alone is app arently insufficient to predict the bird's response to lunar condition. Mor e data on activity at different times of the year combined with measurement s of prey availability and information on the timing and identity of predat ors are needed to allow a definitive explanation for the influence of lunar light on these birds.